1. Field of the Invention
This invention relates to an image forming apparatus such as a copier or a printer using the electrophotographic method or the electrostatic recording method.
2. Related Background Art
FIG. 10 shows an example of an image forming apparatus.
In the image forming apparatus of FIG. 10, when a copy (image formation) starting signal is inputted, a photosensitive drum 1 is uniformly charged to a predetermined potential by a charging device 3. On the other hand, a unit 9 comprising an original irradiating lamp, a short-focus lens array and a CCD sensor constructed integrally with one another scans an original G placed on an original supporting table 10 while irradiating the original G, whereby the reflected light of the irradiating scanning light from the surface of the original is imaged by the short-focus lens array and enters the CCD sensor. The CCD sensor is comprised of a light receiving portion, a transferring portion and an output portion. In the light receiving portion of the CCD, an optical signal is changed into a charge signal, which is sequentially transferred to the output portion by the transferring portion in synchronism with a clock pulse, is converted into a voltage signal in the output portion, is amplified and made into low impedance, and is outputted. An analog signal thus obtained is subjected to well known image processing and is converted into a digital signal, and is sent to a printer portion. In the printer portion, an electrostatic latent image corresponding to the image of the original is formed on the surface of the photosensitive drum 1 by laser exposing means 2 which scans the light L of a solid state laser element emitting light in response to the above-mentioned image signal by a rotatable polygon mirror rotated at a high speed.
Next, this electrostatic latent image is developed by a developing device 4 containing therein a so-called two-component developer having toner particles and carrier particles, whereby a toner image is formed on the photosensitive drum 1.
The toner image formed on the photosensitive drum 1 in this manner is electrostatically transferred onto a transfer material P by a transfer device 7. Thereafter, the transfer material P is electrostatically separated from the surface of the photosensitive drum 1 and is conveyed to a fixing device 6, whereby the toner image on the transfer material P is thermally fixed, and the transfer material P is discharged out of the main body of the image forming apparatus.
On the other hand, after the transfer of the toner image, any toner not transferred to the transfer material P but remaining on the surface of the photosensitive drum 1 (residual toner after transfer) (untransferred toner) is removed by a cleaner 5, and any charge on the photosensitive drum 1 is eliminated by a pre-exposure device 8, and the photosensitive drum 1 is used for the next image forming process.
Also, in recent years, there has appeared a cleanerless image forming apparatus in which the cleaner 5 is detached and cleaning simultaneous with development is effected by the developing device 4. The cleaning simultaneous with development is a method of collecting the residual toner after transfer remaining on the photosensitive drum 1 after the transfer into the developing device 4 by a fog removing bias during the development at the next and subsequent steps. Thereby, the residual toner after transfer is collected and is also used after the next step and therefore, any waste toner can be made null. Also, the merit in terms of space is great and the apparatus can be made greatly compact.
Further, from merits such as low ozone and low electric power, a contact charging device, i.e., a device of a type in which a charging member, to which a voltage is applied, is made to abut against the photosensitive drum 1 as a member to be charged to thereby charge the surface of the photosensitive drum 1 has been put into practical use as charging means for the photosensitive drum 1.
As the contact charging device of such a type, a device of the magnetic brush type (hereinafter referred to as the magnetic brush charging device) is preferably used from the viewpoint of the stability of charging contact.
In the magnetic brush charging device, electrically conductive magnetic particles are directly magnetically restrained on a magnet or on a sleeve containing a magnet therein to thereby construct a magnetic brush as a charging member, and the magnetic particles is brought into contact with a member to be charged in its stopped state or its rotated state, and a voltage is applied thereto to thereby effect charging.
Also, a member comprising electrically conductive fibers formed into a brush-like shape (hereinafter referred to as the "fur brush") or an electrically conductive rubber roller comprising electrically conductive rubber made into a roll-like shape is preferably used as a contact charging member.
Particularly, when such a contact charging member is used and as a member to be charged, use is made of an ordinary organic photosensitive member having a surface having electrically conductive fine particles dispersed therein, or an amorphous silicon photosensitive member or the like, it is possible to obtain on the surface of the member to be charged a charging potential substantially equal to a DC component of a bias applied to the contact charging member. Such a charging method is called injection charging. According to this injection charging, the charging of the member to be charged does not utilize the discharging phenomenon when a corona charger is used and therefore, completely ozoneless and low electric power consumption type charging becomes possible, and attention has been attracted thereto.
However, when such an image forming apparatus is not provided with a cleaner and the collection simultaneous with development by the developing device 4 is effected and image formation is repeated, if the residual toner after transfer cannot be completely collected by the developing device 4, a so-called "positive ghost" in which the previous image remains thinly has sometimes occurred. This positive ghost is a phenomenon occurring because when the surface of the photosensitive drum 1 passes the contact charging member, that portion of the surface of the photosensitive drum 1 which is under the residual toner after transfer, i.e., to which the residual toner after transfer adheres, cannot be charged, and it will become more remarkable when the contact charging member is contaminated.
So, in order to charge this portion which is under the residual toner after transfer during charging, it becomes important to once scrape off the residual toner after transfer from the surface of the photosensitive drum 1 during charging, and return it to the surface of the photosensitive drum 1 after charging, and thereafter collect it by the developing device 4.
In this case, means for causing an electrically conductive brush 11 as shown in FIG. 11 of the accompanying drawings to abut as auxiliary means against the portion on the surface of the photosensitive drum 1 between the transfer device 7 and the charging device 3, and applying a bias opposite in polarity to the charging bias by the charging device 3 to thereby make the residual toner after transfer easy to introduce into the charging device 3, and preventing the aforedescribed positive ghost from occuring. In FIG. 11, the reference numeral 31 designates a magnet roller (magnetic particle bearing member) rotated in the direction of arrow R3, the reference numeral 32 denotes a regulating blade, the reference numeral 33 designates a magnetic carrier (magnetic particles) borne on the surface of the magnet roller 31 and contacting with the surface of the photosensitive drum 1, and the reference numeral 34 denotes a charging bias applying power source for applying a charging bias to the magnet roller 31.
However, when the repetition of image formation or image formation of high image percentage is effected or when transfer efficiency is aggravated in a low humidity environment or the like, there has been the problem that the toner adheres to the end portion of the electrically conductive brush and the toner gets mixed in the charging device 3 or drops onto the transfer material P. There has also been the problem that by the rotation of the photosensitive drum 1, the electrically conductive brush becomes gradually inclined and interferes with the charger 3 to which a bias of the opposite polarity has been applied.